Statements of fact are the primary building blocks of reasoned arguments.
Premises can be simple statements of "fact." For example: "The Earth circles the Sun," or "The United States is on the Earth."
You could make the argument:
PREMISE: The Earth circles the Sun
PREMISE: The United States is on the Earth
CONCLUSION: Therefore, the United States also circles the Sun.
Premises can also be more complex or conditional statements. For example "If the sun comes out, the snow will melt." Factual information is the primary source of evidence for scientific arguments (e.g. McNeill, 2006).
However, premises are NOT necessarily true! "The Sun circles the Earth" is a statement that people who lived before Copernicus agreed on (and is consistent with visual observations). The statement "The Sun circles the Earth" is a reasonable premise for an argument, even though we now know that the statement is incorrect. Therefore, for our purposes a "premise" refers to a piece of information that IS part of a reasoned argument, but does NOT necessarily imply that the information is true.
Although premises do not need to be true to construct arguments, scientific arguments will only be strong if based on true premises.
When are premises most likely to be true?
Ultimately, scientific truth is based on repeated, objective measurements. "Objective" measurements are where the measurements themselves do not depend on human judgment or interpretation (therefore qualitative research, case studies, professional opinions, etc. do not provide strong premises for science; Ebell et al., 2004). Quantitative research separates data collection (which strives to be objective) and data interpretation (which depends on reason and judgment).
Repeatability means that measurements are "reliable": able to be measured by any researchers who make the measurements appropriately.
There are three primary sources of repeated, objective measurements in science:
1) Premises based on data that you collect YOURSELF.
2) Premises based on data that OTHER people have reported and subjected to rigorous peer-review.
3) Premises that are the CONCLUSIONS of valid or strong arguments (i.e. well-reasoned arguments based on true premises):